Microelectromechanical systems (“MEMS” or “MEMS devices”) are used in a growing number of applications. For example, MEMS devices often are implemented as gyroscopes to detect pitch angles of airplanes, and as accelerometers to selectively deploy air bags in automobiles. In simplified terms, such MEMS devices typically have a fragile structure suspended above a substrate, and associated circuitry that both senses movement of the suspended structure and delivers the sensed movement data to one or more external devices (e.g., an external computer). The external device processes the sensed data to calculate the property being measured (e.g., pitch angle or acceleration).
To protect their fragile structure, MEMS devices typically have some type of protective apparatus, such as a package. Specifically, MEMS devices often have a package that seals the structure within a protective chamber. The package often is a first, second, or third level package. If the package is properly sealed, environmental contaminants should not interfere with or damage the structure. Some MEMS devices also seal a gas within the chamber to further optimize device performance.
Many MEMS devices are packaged to have one cavity—i.e., a single cavity containing a one or more suspended masses. Others, however, have multiple cavities with one or more masses that have different functions. As such, those different cavities may have different pressure requirements. For example, a first cavity having a low-G accelerometer may operate better under a vacuum or low pressure, while a second cavity (of the same MEMS device) having a high-G accelerometer may perform better at atmospheric or higher pressures. Efficiently and effectively fabricating the MEMS device with different pressures has been a continuing challenge.